Method of forming electrical insulating film on aluminium metals



TAKASHI SUZUKI 3,531,385

METHOD OF FORMING ELECTRICAL INSULATING FILM ON ALUMINIUM METALS Sept. 29, 1970 Filed Dec. 13, 1968 0 VOLTAGE 0F BA TH 0 VOLTAGE 0F 5/) TH INVENTOR 5 LL 2 LLK/ Tmmsm ATTORNEYS United States Patent U.S. Cl. 204-58 2 Claims ABSTRACT OF THE DISCLOSURE Method of forming an electrical insulating film on a surface of aluminum metal (aluminium and its alloy) characterized by carrying an anodic oxidation in a spark discharge range of an electrolytic voltage.

The present invention relates to a method of forming an electrical insulating film on the surface of aluminum metal including aluminium and its alloys.

When an electrolytic process is performed in an aqueous solution of sulfuric acid, chromic acid, oxalic acid or the like in use of an aluminum or its alloy as the anode, a porous anodic oxidation film is formed on the anode. This film has a superior electrical insulation characteristics, so that it can be used as a heat-resistant electrical insulation material. In such an anodic oxidation process, the anode current intensity increases as the electrolytic voltage increases. When the voltage is increased to a certain value, a spark discharge is initiated. Hitherto, it is believed that the electrical discharge, on the anode is due to the destruction of insulation of the film. Therefore, an anodic oxidation has not been carried out in the electrolytic voltage range in which the film may be destructed.

The invention has found that an anodic oxidation film having excellent insulation characteristic can be formed speedy under a specific condition even if the electrolysis is carried out within the voltage range in which the electric spark discharge is generated.

According to the present invention, a method of forming an electrical insulating film on a surface of an aluminum metal selected from aluminium and its alloys, the method comprising an anodic oxidation carried out in an electrolyte such as a aqueous solution of sulfuric acid, chromic acid, oxalic acid and the like using said aluminum metal as the anode with a voltage of such a value that will cause an electric spark discharge on said anode, wherein the ratio of the surface area of the cathode to the surface area of the anode is kept at least 1.

FIG. 1 is a diagram showing the relationship between the bath voltage and the bath current during an anodic oxidation, and,

FIG. 2 is a diagram showing the relationship between the bath voltage and the coating forming rate at the anode in the anodic oxidation as shown in FIG. 1.

The relationship between the bath voltage and the bath current in an anodic oxidation, in which the ratio of the surface areas of the cathode to the anode is selected to A conventional anodic oxidation is carried out within the region between the points shown by A and B. As soon as the bath voltage increases to the value indicated as V0, a spark discharge is initiated at the anode with the rapid increase of the bath current from 10 to 1,. The ratio I /Io is about 10. The voltage V0 is referred to as the discharge start voltage. During the spark discharge a noise is heard from the anode and when observed at a dark place the anode is seen clear by small sparks. The voltage V0 can be obtained by observing this phenomenon. As the voltage further increases from V0, the bath current increases along the line CD. In FIG. 2, the relationship between the bath voltage and the formation rate of the film is shown, which the film is obtained under the conditions same as the conditions of the electrolysis shown in FIG. 1. The formation rate of the film shown in FIG. 2 has the same tendency as the bath current shown in FIG. 1. The spark discharge start voltage V0 varies in accordance with the composition of the electrolyte liquid, liquid temperature, and agitation of liquid. Particularly, in case such electrolyte is used as an aqueous solution of sulfuric acid, phosphoric acid or the like having a high dissolving power for the anodic oxide film, the characteriStic as shown in FIG. 2 cannot be obtained unless an intense agitation is given to the bath. When no agitation is given to the bath, the film cannot be formed. When the film formed under voltage higher than the spark discharge starting voltage V0 is observed by a microscope, it is found that small recesses, each having the diameter of about 1 to 2 exist in places on the surface of said film. These recesses may be supposed as spark discharge marks. However, the depth of said recess is not so deep that they affect the electric characteristic of the film. When the voltage is raised more than 50 volts from the spark discharge start voltage Vo, the spark discharge marks become large, and the large marks are formed on projected portions on the surface of aluminum or its alloy forming the anode. Said large marks decrease electrical insulation characteristics of the film in said portions of the aluminum or its alloy. This is true irrespective of the composition of electrolyte bath. Therefore, in order to obtain a uniform coating of the film even on the projected portions of the anode, the anodic oxidation must be carried out with the voltage within the range between the spark discharge start voltage V0 and a voltage higher than said spark start voltage by 50 volts. If the ratio of the surface area of the cathode to that of the anode is decreased than 10:1, the spark discharge start voltage V0 in the same electrolyte increases, and the film obtained under the voltage greater than V0 is not uniform in thickness. Thus, the electric characteristics of the film are degraded. This tendency is true irrespective of the compositions of electrolyte and electrodes.

Example An aluminum sheet of 0.5 mm. thick and 99.7% of purity was treated by anodic oxidation process under various conditions. The results are shown in the table un' 10: l, is shown in FIG. 1. dermentioned.

TABLE Film Thickforming ness Withstand Insulation rate, of the voltage, resistance Conditions of electrolysis n/mtn. film, u v./u (2-0111 Sample No.2 L

1 8% aqueous solution of oxahc acid, 30 0.. v. (Vo=60) 19 8 30(28) 4X10 (IXIO 2 15% aqueous solution of sulfuric acid, 20 0.. 45 v. (V0=32) intense agitation- 30 10 37(25) 1X10 (1 10 3 40% aqueous solution of sulfuric acid, 40 0., 58 v. (Vo=41) intense agitation 50 25 1X10 18 of a film, obtained by avv conventional anodic .oxidation In the table, the parenthesized numerical values show the characteristics of the films obtained by an anodic oxidation process carried out at a voltage less than the spark starting voltage. The film forming rate was determined by by dividing the thickness of the film by the time of the anodic oxidation, the thickness of the film being determined by measuring the section of the sample by means of a microscope. The withstand voltage is defined by a value obtained by dividing an insulation breakdown voltage by the thickness, the voltage being measured by applying a direct current between the aluminum portion of the sample and a cylindrical metal electrode of 25 mm. in diameter and 500 g. in weight. The insulation resistance is measured by placing a mercury electrode on a sample piece.

Further, in this experiment, a carbon plate was used as a cathode, and the ratio of the surface area of the cathode to the surface area of the anode was selected to :1. For agitating the sulfuric acid electrolyte, a chemical pump was used and the electrolyte was formed to circulate.

It will be seen from the above description that the process in accordance with the present invention is effective to form at a higher rate an anodic oxidation film having superior electrical insulation characteristics on an aluminum material or an alloy thereof. The present invention is particularly useful in producing an aluminum conductor with electrical insulation coating by a continuous anodic oxidation of long aluminum wire or strand. Further, the corrosion resistant characteristics of the film obtained in accordance with the present invention is as high as that process, so that the present invention can be applied to any field other than electricity. Thus, the present invention has a broad utility.

What is claimed is:

1. In an electrolytic process for forming an electrical insulation film upon an aluminum surface wherein a porous anodic oxidization film is formed upon such surface by the occurrence of an anodic oxidation in an acidic aqueous electrolyte, the improvement comprising (a) applying an anodic potential within a range such that an electrical spark discharge occurs at the anode; and,

(b) providing a surface area ratio of cathode to anode of at least 10 to 1.

2. The process of claim 1, said electrolysis voltage being of a value between the spark discharge starting voltage and up to volts higher than said starting voltage.

References Cited UNITED STATES PATENTS 3,293,158 12/1966 McNeill et a1. 204-56 2,346,658 4/1944 Brennan et al. 20458 FOREIGN PATENTS 263,603 6/ 1910 Germany. 717,015 10/1954 Great Britain.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

